Typically, furnaces of the foregoing type employ downwardly disposed suspended carbon electrodes which gradually erode at the tip. In order to maintain a constant arc length the electrode is gradually lowered further into the furnace. The electrode is constructed of segments, each new segment being attached in situ to the upper end of the existing electrode, as required to maintain the operation of the furnace. The usual method of assembly is to provide each electrode segment with a screw threaded socket at either end. Successive segments are usually joined by a correspondingly screw threaded cylindrical connector, known as a nipple, part of which is screwed into each of the segments joined thereby. Normally when the segments are assembled, according to the prior act, they are screwed together under a generally compressive force.
A problem which has been encountered, particularly in larger installations, is a tendency for the electrode to break, resulting in a serious loss of production every time it has to be replaced. We now believe that a major cause of stress in the electrode, leading to breakages, has in the past been poor connection between the faces of the screw thread on the nipple and the socket respectively. This arises because these members are constructed from various forms of carbon whose mechanical properties make it impractical to machine the thread with sufficient accuracy to ensure good contact between each of the faces. The usual method of assembly, done under a generally compressive force, does not improve this condition because during such an assembly only the outer face of each screw thread will abrade. Particles abraded from these outer faces may collect between the inner faces of the threads further decreasing the area of contact. Yet, it is these inner faces that act as load bearing faces when the electrode is suspended in the furnace. Thus, the weight of the assembled electrode may cause the collapse of the surface of the threads at some of the highly loaded points of contact resulting in separation of the abutting end faces of the successive electrode segments. As a consequence, the whole current then passes through the narrow area of contact between the threads, producing localized overheating, thermal stresses and ultimately, breakage.